The invention relates to a semiconductor device.
In semiconductor devices, the needs for larger-integration design and faster-speed design are high, and the micro-fabrication and faster-speed design of the devices have proceeded by the development of high-accuracy micromachining techniques, the enhancement of electrical characteristics brought about by the adoption of new materials, and the applying of new device structures, etc.
For interconnector-forming processes, as a material which withstands steps performed at a temperature higher than that in the case of aluminum alloys hitherto used and which is less apt to be broken even in a case where the width of an interconnector becomes not more than 500 nanometers, tungsten (W) has come to be used as the material for interconnectors and plugs that establish connection among the interconnectors. Techniques for forming tungsten conductors and tungsten plugs are disclosed in JP-A-10-144623, etc.
When a tungsten film is formed at a temperature not more than 500xc2x0 C. by a sputtering method or a chemical vapor deposition (CVD) method, etc., this temperature is very low in comparison with the melting point of tungsten (about 3400xc2x0 C.) and, therefore, many crystal defects such as vacancies and dislocations often are apt to remain within the grains of tungsten insofar as a period immediately after the film forming is concerned. The vacancies and dislocations make the states of atoms unstable and provide diffusion paths within grains. For this reason, when subjected to a heat hysteresis at a temperature not less than the film-forming temperature, the larger the number of defects such as the vacancies and dislocations, the more the tungsten atoms tend to diffuse, and consequently, the film often becomes dense and contracts in the course of the diffusion of the tungsten atoms migrating to stable locations.
Further, when a tungsten film is formed at a temperature not more than the above-mentioned 500xc2x0 C., the grain size of tungsten often becomes about 50 to 200 nanometers. When fine tungsten conductors each having a width not more than 200 nano-meters are formed by performing the dry etching of the tungsten film, the tungsten conductor width and the tungsten grain size become almost equal to each other. As a result, many grain boundaries are formed in the directions crossing the tungsten conductor, so that there occurs such a structure called xe2x80x9cbamboo structurexe2x80x9d as grains are present in the shape of chain. Grain boundaries are one of the locations where atoms are most apt to diffuse. Thus, the interconnector of the bamboo structure is one of structures in which the breaks of the tungsten conductors are most apt to occur when the atoms diffuse actively within the tungsten conductors and when the film contraction occurs.
In a conventional tungsten conductor-forming step, the temperature of heat hysteresis after forming film was set at a temperature not more than about 500xc2x0 C., and tungsten atoms were not very thermally activated, so that tungsten atoms did not diffuse actively. Besides, because the width of a tungsten conductor was larger than a tungsten grain size, the tungsten conductor was less apt to cause the bamboo structure. Accordingly, in the conventional interconnector-forming processes, the interconnectors were not broken.
However, since the width of the tungsten conductor is miniaturized to be not more than 200 nanometers, the possibility that the structure of the tungsten conductor becomes the bamboo structure is raised. Besides, in a case where a thermal load of not less than 600xc2x0 C. is applied to the tungsten conductor as in the step of crystallizing amorphous tantalum oxide (Ta2O5) for forming a dielectric film of a capacitor or as in the step of oxidizing the lower electrode surface of polycrystalline silicon of the capacitor, there come to occur such cases as the tungsten conductor is broken due to the diffusion of tungsten atoms caused during such high-temperature steps. The smaller the width of the tungsten conductor and the higher the heat treatment temperature, the more the tungsten conductor breakage are apt to occur. It has been found that the tungsten conductor breaks are particularly remarkable when tungsten films are directly deposited on a silicon oxide film.
The object of the invention is to provide a semiconductor device having highly reliable tungsten conductors in which the above problems are solved and defects such as no break of the tungsten conductor occurs.
The reasons for the breakage of the tungsten conductor are as follows:
(1) Because the temperature at which the film is formed is low in comparison with the melting point of tungsten, crystal defects (unstable arrangement of atoms) such as vacancies and dislocations are apt to remain in the interior of the tungsten conductor. This tendency is especially remarkable when the tungsten interconnector is formed directly on the silicon oxide film.
(2) Because a heat hysteresis of a high temperature exceeding the film-forming temperature is applied to the sparse tungsten conductor in which many crystal defects remain, tungsten atoms are apt to diffuse also within grains in addition to the diffusions that occurs on the surfaces of the tungsten conductor and that occurs at the grain boundaries.
(3) Because the tungsten conductor width is equivalent to the grain size of tungsten or is not more than it, the tungsten conductor comes to have the bamboo structure, so that the tungsten conductor is broken even when one of the tungsten grain boundaries is opened.
To solve the above problems, there is provided a semiconductor device of the invention that has the following features.
At least one of the above problems is solved by the following constitution of the invention.
(A) To suppress the surface diffusion of tungsten atoms within the above tungsten conductor and, at the same time, to suppress the diffusion within the grains by lowering the proportion of the unstable arrangement of atoms remaining within the tungsten conductor, a molybdenum (Mo) film (a first electrically conductive film) and another molybdenum film (a third electrically conductive film) are formed, respectively, at the interface defined between the tungsten conductor (a second electrically conductive film) and an interlayer dielectric film (a first dielectric layer), which serves as an underlayer, and on the surface side of the tungsten conductor, whereby the tungsten conductor is sandwiched between the two molybdenum films.
Molybdenum has a melting point lower than that of tungsten although it has a lattice structure similar to that of tungsten. Therefore, crystal defects are less apt to occur in molybdenum than in tungsten. When a molybdenum film having this characteristic is used as the underlayer of the tungsten conductor, tungsten atoms deposit along the arrangement of tungsten atoms of the underlayer, so that a dense tungsten film having few crystal defects can be easily obtained. For this reason, even in a case where the tungsten conductor undergoes a heat hysteresis of a temperature not less than 500xc2x0 C. at a later step, the diffusion within the grains or at the grain boundaries is suppressed and no breakage occurs in the tungsten conductor.
(B) To prevent the bamboo structure of the tungsten conductor from occurring, a molybdenum film (a fourth electrically conductive film) is formed so that it may partition the tungsten conductor into at least two layers (the second electrically conductive film and a fifth electrically conductive film) in the direction of the film thickness.
The tungsten conductor comes to have a two-layer structure, so that the probability of occurrence of tungsten conductor breakage becomes very low even if the breakage at grain boundary occurs in the tungsten conductor of the first film, because electrical connection is established by the other film.
(C) To suppress the surface diffusion of tungsten atoms within the tungsten conductor, a molybdenum film (a sixth electrically conductive film) and another molybdenum film (a seventh electrically conductive film) are formed at the interface defined between the tungsten conductor (the second electrically conductive film) and the underlayer, and on the surface side_of the tungsten conductor, respectively, whereby the tungsten conductor comes to be covered with the molybdenum films.
By making the surface, interface and side of the tungsten conductor be in contact with molybdenum having a lattice space similar to that of tungsten, the surface diffusion is suppressed and no grain-boundary break of a tungsten wire comes to occur.
Incidentally, in the above features of the invention (A), (B) and (C), it is not always necessary that the molybdenum films be made of pure molybdenum, that is, they may be films made of a material having a lattice space similar to that of tungsten, and the use of any material having the effect of suppressing the diffusion of tungsten atoms is usable. The films may be made of, for example, any one of pure molybdenum containing not less than 99% Mo by atomic ratio, a molybdenum alloy containing not less than 90% Mo by atomic ratio, molybdenum nitride containing not less than 40% Mo by atomic ratio, molybdenum carbide containing not less than 40% Mo by atomic ratio, molybdenum boride containing not less than 40% Mo by atomic ratio, tungsten nitride containing not less than 40% W by atomic ratio, tungsten carbide containing not less than 40% W by atomic ratio, and tungsten boride containing not less than 40% W by atomic ratio.
The reliability against the breakage of the tungsten conductor is greatly improved by providing a semiconductor device having the above features.
Before the embodiments of the invention are described, the following terms used in this specification are explained. xe2x80x9cthe main elementsxe2x80x9d
The xe2x80x9cmain elementxe2x80x9d used in this specification is defined to be an xe2x80x9celement having the highest ratio of the number of atoms in a material.xe2x80x9d The property of this xe2x80x9cmain elementxe2x80x9d often determines the main property of an obtained material. xe2x80x9cthe main componentxe2x80x9d
In a compound material, when the ratio of the number of atoms of a plurality of elements, which constitute the particular compound, to the total number of atoms regarding a plurality of elements (including impurities and additives) constituting the whole of the material is the highest, the particular compound is defined as the xe2x80x9cmain componentxe2x80x9d in this specification.